First-principles Studies On Li Storage Of Doped Spinel LiNi_0.5Mn_1.5O4and Mg Storage Of MoS₂Nanoribbon

Electrochemical energy storage devices (such as rechargeable lithium/magnesium batteries) have attracted much attention due to their high energy conversion efficiency. The rising of electric vehicles and the smart grid promotes rechargeable lithium/magnesium batteries with the desirable properities such as safety, high energy density, fast charging and discharging, and long cycle life. Therefore, it is particularly imperative to improve the stability, lithium/magnesium storage capabilities, and ion diffusion rate of electrode materials. For rechargeable Li-ion batteries, spinel LiNio.5Mn1.5O4is recently considered as a promising positive electrode material, yet its large-scale application is limited due to relative poor cycling and rate performance. Metal doping is expected to be an effective approach to improve the electrochemical performance of spinel LiNi0.5Mn1.5O4. Ambiguity exists, nevertheless, as to deeper understanding into doping effects on the structural and electrochemical properties of metal-doped LiNi0.5Mn1.5O4electrode materials. On the other hand, MoS2is a typical layered metal disulfide, and S-Mo-S molecular layers are held together through weak Van der Waals interactions. MoS2is regarded as a desirable intercalation host material because of its large interlayer distance. However, as a positive electrode material for rechargeable magnesium batteries, MoS2has been less studied. As the reversible capacity and voltage platform of batteries are mainly determined by the positive electrode materials, we focused on the development of novel materials in the positive electrodes for lithium/magnesium rechargeable batteries.With the development of computational science, study on the properties of energy materials is not confined to the scope of experiments. Using quantum mechanics and other physical laws, first-principles based on density functional theory (DFT) provides practical guidances for the design and analysis of high performance electrode material by calculating the electrode geometry, electronic structure, thermodynamics and electrochemical properties.In view of the importance of positive electrode material for rechargeable lithium/magnesium batteries, this thesis aims to design and analyze Li storage of doped spinel LiNi0.5Mn1.5O4and Mg storage of MoS2nanoribbon. The main contents include:(1) In this work, systematic first-principles studies based on the density functional theory have been carried out to investigate electronic and structural properties of LiM0.125Ni0.375Mn1.5O4(where M=Mg, Cr, Fe, and Co) cathode. It is found that doping a small quantity of metal M atoms into the Ni site results in a decrease in the volume variation during the lithiation/delithiation cycle (the volume variations for Mg-doping, Cr-doping, Fe-doping and Co-doping are2.4%,4.1%,3.8%and4%from lithiated phase to delithiated phase respectively, whereas4.7%for the undoped case). Electronic calculations suggested that transition metal doping (Cr-, Fe-, and Co-doping) would effectively improve the electronic conductivity of the system. To evaluate effects of dopants on lithium mobility, we calculated the activation energies for lithium diffusion in M-doped LiNi0.5Mn1.5O4cathode. Our calculations indicate that doping with Co can potentially reduce lithium diffusion barrier as compared to that of pristine LiNi0.5Mn1.5O4spinel.(2) Based on density functional theory, the feasibility of the zigzag single-layer MoS2nanoribbon as the positive electrode material for rechargeable magnesium batteries is systematically investigated. The results showed that Mg atoms tend to occupy the top sites above Mo atom at the edge of the nanoribbon. With the increase in the number of adsorbed Mg atoms, the MoS2nanoribbon (Nz=5) could accommodate up to6Mg atoms at both sides. According to the Faraday law, a maximum theoretical capacity of200.9mA h/g could be attained. Electronic analysis showed that the chemical bonds between the adsorbed Mg atoms and MoS2nanoribbon is ionic bonds, while there is a certain degree of covalent component. Using CI-NEB method, the Mg diffusion path on MoS2nanoribbon was determined. The activation barrier is only0.48eV, which shows a great decrease compared to that in the bulk MoS2(2.61eV). This indicates that Mg diffusion kinetics has been greatly strengthened. In summary, the zigzag single-layer MoS2nanoribbon is a promsing positive electrode material for magnesium secondary batteries.